What do i need to add to my cone 6 stoneware clay to make it suitable for slipcasting? I have a trash can full of waste clay that is not worth my time to wedge.

I usually buy slip when I need it. I never make it. However you can 'Google' 'Paul Wandless slip making'. He has a method of making your moist clay into casting slip. You may get the same info here on CAD; try a search.

John 3:16
"For God so loved the world that He gave His only begotten Son, that whosoever believeth in Him should not perish but have everlasting life".

You can make most clays into a slip body but it will not be the best slip body-I suggest if you want trouble free slip results us a slip body made for slipping. Waste clay is not great slip clay-You can use it but it will take some time and test results it may work out and it may not. I have either made slip clay from scratch dry materials or bought slip clay in dry state but never tried to make whatever I was throwing into a slip body.
Mark

John B. Kenny's Complete Book of Pottery Making has directions for making casting slip that involves a 10 day process. This is an old book from the 1950s.
It begins with a dry powder clay recipe plus diffloculent additives and proper blending of 10 minutes three times /day for 10 days. I used this process and it was very successful . I was able to make slip for particular clay bodies.